This invention relates generally to fuel injection pumps of a type adapted to deliver discrete charges of pressurized fuel to the cylinders of an associated internal combustion engine. More particularly, this invention relates to an adjustable spill control system for diverting a portion of a pressurized fuel charge to supply an accurately quantified fuel charge to the associated engine.
Fuel injection pumps which employ a pair of plungers reciprocating in a diametral bore of a rotatable rotor to generate pressurized charges of fuel for delivery to the cylinders of an associated internal combustion engine are well known. Such fuel injection pumps conventionally employ a cam ring having a plurality of lobes which are operatively engageable with the plungers for reciprocating the plungers upon rotation of the rotor. The timing of the reciprocating plungers is controlled by adjusting the angular position of the cam ring. A number of systems have been advanced for regulating the size of the injected fuel charge by means of diverting a portion of a pressurized fuel charge from the fuel distributor path which leads from the pump chamber to discharge passages in the rotor for ultimate distribution to the internal combustion engine. Such mechanical spilling systems have employed either angularly or axially acting spill collars or spill valves within the fuel injection pump to divert fuel to the interior of the pump. An example of a spill control system is exemplified in U.S. Pat. No. 4,376,432 entitled "Fuel Injection Pump With Spill Control Mechanism" wherein a spill collar is mounted adjacent to the cam ring. The spill collar includes a spill port for diverting fuel flow from the charge pump upon registration of a spill passage formed in the rotor with the spill port of the spill collar. Means are provided for adjusting the angular position of the spill collar relative to the cam ring to control the quantity of diverted fuel.
Conventional spill control systems have proved problematical because of difficulties in independently controlling the fuel delivery and the timing advance. The latter difficulties arise from the mechanical connection via the conventional cam ring between the fuel delivery means and the timing advance means. One solution employs a longer cam ramp. However, the latter solution tends to increase dead volume trapped between the pumping plungers. The dead volume is especially critical at high speeds when earlier advances are required and higher pressures generated. Of course, additional filling of the pump chamber is required with the longer ramp. The present invention is a new and improved spill control system which allows for independent control of fuel delivery and timing advance in the fuel injection pump.
An additional problem associated with the incorporation of a spill control system into a fuel injection pump, especially pumps employed for diesel injection applications, results from the wide range of fuel spilling velocities and in particular the potential adverse operational characteristics associated with relatively high fuel spilling velocities and relatively low fuel spilling velocities. In general, the spilling velocity of the fuel is a function of the pressure differential between the pressure of the fuel which is to be diverted and the pressure of the environment into which the fuel will be diverted. High spilling velocities which result from high pressure differentials create cavitation in the injection pump system. However, relatively low spilling velocities retard the nozzle needle closing which may create additional problems regarding emissions and fuel consumption. During the course of operating a fuel injection pump, obtaining the optimum compromise between high and low spill velocities is difficult because the pressures generated by the fuel injection pump vary with the pump speed. The spill control system of the present invention provides a new and improved means for controlling the spill velocity in accordance with the speed variations of the pump.